I think the video is playing behind your WKWebview, could you please debug the view hierarchy and post it? Xcode->Menu->Debug->View Debugging->Capture View Hierarchy when playing the video.

I've tried with a new swift3 project and with your code, there is no problem, here is the view hierarchy:

You can see that from left pannel, the AVPlayerView is in another UIWindow, different from WKWebView,So I guess the UIWindow which contains the WKWebView in your project has a higher windowLevel so it shows above the UIWindow which contains AVPlayer.

And by making the default UIWindow a higher WindowLevel(UIWindowLevelAlert),I reproduced what you've seen in your project.